1. Field of Invention
The present invention relates to a package for a semiconductor light emitting device.
2. Description of Related Art
Semiconductor light emitting devices such as light emitting diodes (LEDs) are among the most efficient light sources currently available. Material systems currently of interest in the manufacture of high brightness, high power LEDs capable of operation across the visible spectrum include group III-V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials; and binary, ternary, and quaternary alloys of gallium, aluminum, indium, and phosphorus, also referred to as III-phosphide materials. Often III-nitride devices are epitaxially grown on sapphire, silicon carbide, or III-nitride substrates and III-phosphide devices are epitaxially grown on gallium arsenide by metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial techniques. The devices include an active, light emitting region sandwiched between an n-type region and a p-type region. Electrical contacts are provided on the n- and p-type regions.
FIG. 1 illustrates a package for an LED, described in more detail in U.S. Pat. Nos. 6,204,523 and 6,274,924. LED die 2 is enclosed in a package which generally includes a bed arrangement 3 upon which the LED die 2 rests, and an optically transmissive cover, generally including a lens. The bed arrangement 3 supports the LED die 2. The bed arrangement 3 includes a lower housing member 4 which has a die placement area 6. The die placement area 6 may be substantially flat, or may be configured as a receptacle. A reflective surface 8 may be provided on the die placement area 6, to direct emitted light outward. A substrate member 10 may be positioned inside the die placement area 6, to support the die 2 itself. The bed arrangement 3 also includes a lead support member 12, which is positioned over the lower housing member 4. Heavy leads 14, provided on the exterior of the package for incorporating the package into circuits and systems, are coupled through the lead support member 12 to fine leads (not shown), which couple directly to the LED die 2. The lead support member 12 includes an aperture 16, in which part of the lower housing is positioned. An optically transmissive cover 18 is positioned over the bed arrangement 3, to cover and protect the LED die 2 and its leads. The cover may be configured to operate as a lens to direct light in an application specific manner.
The cover 18 is made of one or more materials which are chosen for light-transmissive properties and for stability over the environmental conditions under which the LED is to operate. Conventionally, the optically transmissive cover 18 has been made of hard optical materials such as PMMA, glass, polycarbonate, optical nylon, transfer molded epoxy, cyclic olefin copolymer, rigid silicone, other optical plastics, glasses, or other transparent materials.
The design of FIG. 1 necessarily forms a cavity between cover 18 and LED die 2. The cavity sometimes contains air. This air-filled cavity creates one or more abrupt change in refractive index at the refractive index interfaces in the optical path; for example, at the interface between die 2 and the air in the cavity. These index steps tend to trap, deflect or refract, and scatter the light, reducing the percentage of the light generated within the LED chip that is extracted from the package.